Manufacture of alpha cement or plaster from gypsum



June 24, 1930. F, 1,766,448

MANUFACTURE OF A CEMENT OR PLASTEIR FROM GYPSUM Filed Feb. 20, 1925 2 Sheets-Sheet l F. MULLIGAN June 24, 1930.

Filed Feb. 20 1925 2 Sheets-Sheet 2 F/WMZZJ Q y g 22 Patented June .24, 1930 I UNITE-D7: STA

This invention relates toa process of heat treatment of gypsum whereby in one heating operation a cement or plaster product thereofmay be obtained which has improved -properties as hereinafter described, While retaining the shade of whiteness peculiar to products 'of gypsum, such as plaster of Paris, stucco, cement plaster, hard-finish plaster, flooring plaster and the like.- When 10 using the expression gypsum I wish to include notonly natural gypsum and gypsum earths but. also artificial gypsum. There are ,in existence several varieties of theso-called hydraulic cement, of' which =Keenes and Parian cement and flooring plaster are" examples. These are produced by various methods of heating and calcining raw gypsum and by adding to the gypsum either before, during, or after such calcination, various chemicals, principally ,salts of the metals sodium, potassium and iron. The addition of these chemicals is for the express purpose of accelerating the set The process hereinafter described is calculated 'to produce from raw gypsum, 39

' substances such as required for the production of the qualities hitherto known and associated with gypsum cements or plasters made by other processes. p

In the known processes employed in the manufacture of cements and plasters, it is usual to refer to the heat treatment as being 40 Thus, in the manufacture of plaster of Par is, it is usual to refer to calcination of thegypsum up to 400? F.; in the manufacture of Keenes cement as heating to 500 C. (932 F.); and so on; In some casesthe calcination is referred to very roughly as being carried on at a full red heat; at a glowing heat; at a dull red heat; and so on. This method of defining the heat treatment is-not sufficient-1y definite in cases TES " runners MULLIGAN, on BELFAST, I ELAND the'character of the treated as indicated, a product wherein re-;- side the improved qualities h'ereinafterre, ferred to without the addition of other carried on at or up to a certain temperature;

PATENT murncrunn onA cnmniw on PLASTER FROM GYPSUM- Application filed February 20, 1925, Serial No. 10,549, and in- G reat Britain May 28, 1924.

OFFICE "where a certain critical condition of the especially when the gypsum is calcined in a closed kiln in conjunction with the fuel, that the product will vary in accordance -with gypsum and the nature of the fuel. v Ihave carried out numerous experiments and I have found that if gypsum, in lumps and in conjunction with the fuel, is calcined,

slowly, in a kiln for a prolonged period of time itcan be brought ,to. a critical stage, when its crystalline structure begins to break down, this; breaking down 'being indicated by the development on the surface of the chemical composition, on analysis, agrees with that of calcium sulphate, I, have found that, during the slow calcination, up to this a stage, certain, chemical and physical reactions (which 'can' not readily be followed) have" taken place which areof such a nature that the calcined gypsum taken at this stage, afterit is allowed to cool naturally, or is otherwise suitably cooled, and is then pul- The fuel in thekiln is carefully proporh ,tioned to the gypsum so that when this stage the crystalline structure of the gypsum be preserved but as the appearance of efl'lorescence clearly indicates that the critical stage i is reached the heatfa lls gradually, resulting in a slow cooling action. It is desirable that has been reached, I earry on the calcination until incipient efilorescence actually appears,

as by so doing'such appearance clearly indicates to the workman that the material hasbeen sufliciently heat treated and provides an index that insures uniformity of the product. The actual small amount of sur face eiflorescen'ce produced results in a slight" loss but this loss is negligible. As a preliminary step, it is necessary to 0'5, lumps of, a mineral e'fllorescence whose 75 verized, forms a 'white cement or plaster ments of trial and error on a small scale,

the correct ratio of fuel to gypsum, which will cause calcination to the stage of incipient efllorescence to take place and the combustion then to cease, can be found for a particular case.

In carrying out .the invention I prefer ably use a kiln such as shown in the drawingsl, and I build up the fuel, preferably a goo North of Ireland lump gypsum in alternate layers,-the kiln charge being say five tons of gypsum and twelve and one-half hundredweights of coal, i. e., a ratio of fuel to gypsum of one to eight, which ratio of fuel I have found to be sufficient to generate in the aforesaid kiln a heat sufficient to effect the chemical changes hereinbefore referred to within a period of ten to fifteen hours. After-the calcination to the incipient efiiorescence stage has been completed, the mass is allowed to cool naturall or is cooled, and is thereafter pulverized by any suitable machinery, when it forms the white hydraulic cement or plaster aforesaid.

The kiln WhlChI prefer to use and which is shown by way of example in the drawing is of such construction that the calcination 'is carried on slowly, and the hot gases, en-

erated by the combustion of the fuel, t oroughlpy permeate the gypsum which, after being dehydrated, is exceedingly porous, and

these highly heated gases acting during the preliminary period of calcination produce the chemical and physical chan es in the character of the gypsum. Accor ing to the nature of the gypsum being treated and the calorific value of the fuel, the fuel. ratio niaybe slightly increased or diminished, but the ratio of fuel must not be increased to such an extent as to cause sintering or clinkering of the mass, conditions which have been found to be harmful to the product.

Fig. 1 is an elevation; Fig. 2 a vertical section on the line 22 Figs. '1 and5-; Fig. 3 a sectional plan on the line 3-3 Fig. 2; Fig. 4 a sectional plan on the line 4-4 Fig. 2; and Fig. 5 a section (looking upwards) on the line 55 Fig. 2.

As will be seen the kiln has a cylindrical wall a with lining b of firebrick, and firebrick dome top asupported by metal stays d.

. e is the outlet flue, f the floor grate, g the air-inlet and m the fire grate. It will be seen the front is left open at it so that the fuel and gypsum can be conveniently and carefully'stacked upon the floor grate in layers indicated at z', 7', and after the stacking is completed the front is closed by fireclay slabs is. It will be seen the outlet flue quality of Scottish splint coal andv '48 hours is allowed for cooling.

e is small compared with the capacity of the kiln and the floor grate area so that slow combustion is assured. The starting fire is placed on the grating m. For the purpose of controlling the heat a damper 0 may be provided. I

The following may be taken as a typical example of the carrying out of the process.

The kiln is about 9 feet high and the opening in the front is about 2 feet wide. North of Ireland gypsum in lumps and a good quality of Scottish round coal are charged into the kiln in alternate layers, the charging taking place through the front opening and the proportion of gypsum to fuel being 4% tons of gypsum to 11 hundredweights of coal.

A fire is made with shavings, sticks or the like on the fire-grate which is about three feet below the floor grate which supports the kiln charge. This starting fire is kept burning brightly until the kiln charge is burning well, the front having been previously closed up with the fire-clay slabs k.

The charge burns without much attention forabout 15 hours whereafter-a period of The kiln is then opened and the lumps of calcined ypsum removed, these lumps having a.

slight development of efiiorescenee on their surfaces.

Attempt has been made to ascertain the temperature during the burning but it was found impossible to get the thermocouple of the instrument into a satisfactory position. The highest temperature recorded is about 650 C. but probably the temperature of the properly burning mass is much higher, from 900 C. to 1000 '0. As was pointed out before, a knowledge of the temperature of burning is not required.

If a specially white product is required, the lumps of calcined gypsum are brushed clean with a wire brush. In any case, after burning, the product is fed into disintegrators andthe resultant powder sieved to 200 mesh, at the same time being mixed with ,4 of alum. I

The white powder mentioned'above is known as No. 1, and the ordinary slightly coloured product is known as No. 2 cement.

Microscopic examination of cement prop erly prepared as above definitely shows that it'is of a crystalline structure. Further tests definitely show that all the water of crystallization has been lost during calcination, the

ably calcined in fireclay slabs are removed, giving ready access to the interior, and the lumps of calcined gypsum carefully picked out. Any ash or the like readily falls. away from the lumps. Some of the lumps may have slight local soiling which can be removed by brushing with a stiff brush, or by cutting or scraping the lumps.

ily done owing to the open front of thekiln.

lVhen' the kiln is closed up, by the slabs being fitted into the V shaped opening it, the air can only gain access to the interior of the kiln through the reduced opening 9 whilst the waste products bf combustion escape by the small flue e. 1

If powdered gypsum be used it is preferthe briquetted form.

Although I have described and shown a suitable type of kiln, I do not confine myself thereto.

Alum, nitre, or the like may be added, and it may be associated with the gypsum before calcination, or it may be added in the course of pulverization of the calcined mass or to the finished product. The proportion of such agent may be low,

gypsum being usually suificient.

The cement or plaster produced 'by the plaster herein described has superior properties. It is intensely white, has a good spreading capacity, and sets very hard. Further, it has the property of setting under water which is a valuable characteristic.

Having now fully described my invention what I claim and desire to secure by Letters Patent is 1. A process for the manufacture of a cement or plaster from gypsum, in one heating operation, comprising stacking crude gypsum together with coal in a closed kiln and in about the proportion by weight of one part of coal to eight parts of gypsum, subjecting the gypsum to slow calcination by burning the coal so that in the process of calcination the products of combustion are caused to permeate the gypsum and thoroughly act thereon for such a period oftime as will bring the gypsum to a stage when efilorescence begins to appear, and then cooling the calcined mass.

2. A process for the manufactureof a cement or plaster from gypsum in one heating operation. comprising stacking gypsum in briquette form together with coal in a as little-as one quarter per cent of the weight of closed kiln and in about the proportion by weight of one part of coal to eight parts of gypsum, subjecting the gypsum to slow calcin'ation by burning the coal so that in the process of calcination the products of combustion are caused to permeate the gypsum and thoroughly act thereon for such a period of time as will bring the gypsumto a stage when efflorescence begins to appear, and then cooling the calcined mass.

In testimony whereof I afiix my signature.

FRANCIS MULLIGAN. 

